Intervertebral disc prosthesis having multiple bearing surfaces

ABSTRACT

An intervertebral disc prosthesis comprises a left prosthesis component and a symmetric right prosthesis component positioned in an intervertebral space. Each prosthesis component includes a superior vertebra facing surface, an inferior vertebra facing surface, and a substantially spherical bearing surface. The bearing surface is positioned between the superior vertebra facing surface and the inferior vertebra facing surface. The superior vertebra facing surface is provided on a superior endplate and the inferior vertebra facing surface is provided on an inferior endplate. The superior endplate is operable to rotate relative to the inferior endplate upon the bearing surface. Each prosthesis component further comprises an elastic member positioned between an endplate and the bearing surface. The elastic member may be substantially cylindrical with a plurality of resilient ribs. Flexion/extension, lateral bending, and torsional movement are allowed by a combination of endplate rotation upon the bearing surface and compression of the elastic member.

BACKGROUND

This invention relates to the field of prosthetics, and moreparticularly, to an intervertebral disc prosthesis designed to replace adamaged intervertebral disc.

The human spine consists of twenty-four small bones known as vertebrae,or “vertebral bodies,” that protect the spinal cord and providestability to the torso. The vertebrae are arranged in a column andstacked vertically upon each other. Between each vertebra is a fibrousbundle of tissue called an intervertebral disc. These intervertebraldiscs act as a cushion to the spinal column by absorbing energy andtransmitting loads associated with everyday movement. They also preventthe vertebrae from rubbing against each other.

Each intervertebral disc comprises two distinct regions. A firm outerregion, the annulus, maintains the shape of the intervertebral disc. Aninner region, the nucleus, provides a resilient tissue that enables thedisc to function as a shock absorber. Over time, the normal agingprocess causes the intervertebral discs to degenerate, diminishing theirwater content and thereby reducing their ability to properly absorb theimpact associated with spinal movements. Diminished water content in theintervertebral discs may also cause the vertebrae to move closertogether. Tears and scar tissue can weaken the discs, resulting ininjury. When the discs wear out or are otherwise injured, a conditionknown as degenerative disc disease results. With this condition, discsdo not function normally and may cause pain and limit activity.

The condition of degenerative disc disease can potentially be relievedby a surgical procedure called artificial disc replacement. In thisprocedure, the damaged intervertebral disc is replaced by anintervertebral prosthetic device (i.e., an artificial disc).

Many artificial discs have been proposed in the past. A typical priorart artificial disc comprises two metal endplates, one endplate thatfaces a superior vertebra and one endplate that faces an inferiorvertebra. A bearing surface is provided between the two metal endplates,allowing the endplates to rotate relative to one another and generallymimic the motion allowed by a natural disc.

Although current intervertebral disc prosthetic devices have enjoyedsuccess, it would be beneficial to add additional desirable features tothe prosthetic devices. For example, the large size of many currentintervertebral prosthetic devices requires the devices to be insertedfrom an anterior approach. An anterior approach presents numerouschallenges to the surgeon including the fact that the insertion site maybe near the aorta and vena cava. Therefore, it would be advantageous toprovide an intervertebral disc prosthesis that is relatively small andincludes additional features to facilitate a posterior insertion.

Another advantage would be to provide a prosthetic device that moreclosely mimics the restricted movements offered by a natural disc. Thestructure of the human spine generally allows significant degrees offlexion/extension (and particularly flexion) but allows lesser degreesof axial rotation (torsion) and lateral bending. Therefore, it would beadvantageous to provide an intervertebral prosthetic device operable toallow a significant degree of flexion/extension in a patient while onlyallowing restricted axial rotation and lateral bending.

SUMMARY

An intervertebral disc prosthesis is disclosed herein. Theintervertebral disc prosthesis is designed for implantation between anupper vertebral body and a lower vertebral body. The intervertebral discprosthesis generally comprises a left prosthesis component and asymmetric right prosthesis component.

The left prosthesis component includes a left superior vertebra facingsurface, a left inferior vertebra facing surface, and a substantiallyspherical left bearing surface. The left bearing surface is positionedbetween the left superior vertebra facing surface and the left inferiorvertebra facing surface. The left superior vertebra facing surface isprovided on a left superior endplate. The left inferior vertebra facingsurface is provided on a left inferior endplate. The left superiorendplate is operable to rotate relative to the left inferior endplateupon the left bearing surface. The left prosthesis component furthercomprises a left elastic member positioned between the left superiorendplate and the left bearing surface. In an alternative embodiment, theleft elastic member may be positioned between the left inferior endplateand the left bearing surface.

The right prosthesis component is generally symmetric with the leftprosthesis component. Accordingly, the right prosthesis componentincludes a right superior vertebra facing surface, a right inferiorvertebra facing surface, and a substantially spherical right bearingsurface. The right bearing surface is positioned between the rightsuperior vertebra facing surface and the right inferior vertebra facingsurface. The right superior vertebra facing surface is provided as partof a right superior endplate. The right inferior vertebra facing surfaceis provided as part of a right inferior endplate. The right superiorendplate is operable to rotate relative to the right inferior endplateupon the right bearing surface. The right prosthesis component furthercomprises a right elastic member positioned between the right superiorendplate and the right bearing surface. In an alternative embodiment,the right elastic member is positioned between the right inferiorendplate and the right bearing surface.

The left bearing surface and the right bearing surface are configuredsuch that the center of rotation of the left prosthesis component andright prosthesis component are positioned along a line parallel to alateral midline of the upper or lower vertebral body when the vertebralbodies are in an equilibrium position. In one embodiment, the center ofrotation of each prosthesis component is positioned along a linepositioned to the posterior of the lateral midline of each vertebralbody.

In one embodiment, the left elastic member and right elastic member areboth substantially cylindrical in shape and are both comprised anelastomeric material. The left elastic member and right elastic memberboth include a plurality of resilient ribs operable to deform when athreshold force is applied to the ribs and return to their originalshape when the force is removed. A cavity configured to receive the leftelastic member is formed in the left superior plate. A cavity configuredto receive the right elastic member is formed in the right superiorplate. The left elastic member includes another cavity configured toreceive the left bearing surface. The right elastic member includes yetanother cavity configured to receive the right bearing surface. In oneembodiment the left bearing surface is provided as part of a leftdome-shaped member having a concave bearing surface. The right bearingsurface is provided as part of a right dome-shaped member having aconcave bearing surface. In this embodiment, a left ball-shaped bearingsurface is provided on the left inferior endplate and a rightball-shaped bearing member is provided on the right inferior endplate.The left concave bearing surface rotatably engages the left ball-shapedbearing surface. Likewise, the right concave bearing surface rotatablyengages the right ball-shaped bearing surface. Thus, a ball and socketarrangement is formed on both the left component and the right componentof the intervertebral disc prosthesis.

Flexion/extension movement is primarily allowed in the intervertebraldisc prosthesis by rotation of the superior endplates relative to theinferior endplates upon the bearing surfaces. Lateral bending movementto the left or right is primarily allowed by compression of the leftelastic member or right elastic member when a force is applied to rotatethe superior endplates relative to the inferior endplates. Torsionalmovement is primarily allowed by compression of the left elastic memberand right elastic member when a force is applied to rotate the superiorendplates relative to the inferior endplates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows and exploded side view of a first prosthetic device of theintervertebral disc prosthesis;

FIG. 2 shows a side view of the assembled prosthetic device of FIG. 1;

FIG. 3 shows a perspective view of the prosthetic device of FIG. 2including a view of the elastic member within the superior endplate;

FIG. 4 shows a bottom view of the superior plate of the prostheticdevice of FIG. 1;

FIG. 5 shows the arrangement of the prosthetic device of FIG. 1 and asecond complementary prosthetic device in relation to a vertebral body;

FIG. 6 shows a rear perspective view of the first and second prostheticdevices as arranged in FIG. 5;

FIG. 7 shows a front view of the intervertebral disc prosthesis of FIG.6;

FIG. 8 shows an alternative embodiment of the intervertebral discprosthesis of FIG. 6;

FIG. 9 shows an exploded side view of an alternative embodiment of theprosthetic device of FIG. 1;

FIG. 10 shows an exploded anterior perspective view of the prostheticdevice of FIG. 9;

FIG. 11 shows an exploded posterior perspective view of the prostheticdevice of FIG. 9;

FIG. 12 shows a side view of the assembled prosthetic device of FIG. 9;

FIG. 13 shows the arrangement of the prosthetic device of FIG. 9 and asecond complementary prosthetic device in relation to a vertebral body;and

FIG. 14 shows an alternative embodiment of the intervertebral discprosthesis of FIG. 1 in relation to a vertebral body.

DESCRIPTION

The intervertebral disc prosthesis described herein is comprised of botha first prosthetic device and a complimentary second prosthetic device.The first prosthetic device is first explained herein with theunderstanding that the second prosthetic device is a symmetric/mirrorimage of the first device. Arrangement and operation of the first andsecond prosthetic devices follows the general description of the firstprosthetic device.

First Prosthetic Device

With general reference to FIG. 1, an exploded side view of a firstprosthetic device 20 for an intervertebral disc prosthesis is shown. Thefirst prosthetic device 20 comprises a superior endplate 22, an elasticmember 24, a dome-shaped bearing member 26, and an inferior endplate 28.The superior endplate 22 is configured to retain the elastic member 24,and the elastic member 24 is configured to retain the dome-shapedbearing member 26. The dome-shaped bearing member 26 is configured toengage a spherical bearing surface 30 of the inferior endplate 28.

The superior endplate 22 is generally block-shaped with a trapezoidallongitudinal cross-section. The superior endplate 22 is comprised of arigid material such as a hard metal, ceramic, or hard polymer material.In one embodiment, the superior endplate 22 is comprised of cobaltchromium. In yet another embodiment, the superior endplate 22 iscomprised of multiple materials having different properties. Forexample, the superior endplate 22 may be formed as a multi-durometerelastomer comprising a relatively hard outer portion, a relatively softinner portion, and a relatively hard bearing portion.

As shown in FIGS. 1 and 3, the superior endplate 22 includes an uppersurface 40 designed to face a superior vertebra in an intervertebralspace that is defined by the superior vertebra and an inferior vertebra.An upper keel 42 is positioned on the upper surface 40 and extends fromthe upper surface. The keel 42 is configured to fit in a channel formedin the superior vertebra and secure the endplate 22 to the superiorvertebra. An alignment hole 44 is provided in the keel 42. As will beexplained in further detail below, the alignment hole 42 is designed toassist the surgeon in aligning the first prosthetic device 20 with asecond prosthetic device.

With reference now to FIG. 4, the superior endplate 22 also includes alower surface 50. The lower surface 50 includes a posterior lower tier52 and an anterior sloped portion 54. The anterior sloped portion 54ramps upward from the lower tier 52 at an angle. In one embodiment, theangle between the lower tier 52 and the sloped portion 54 is about 30°.The perimeter of the lower surface 50 defines a right edge 56 that isgenerally parallel to a left edge 58, with the right edge being longerthan the left edge. A curved anterior edge 57 extends between the rightedge 56 and left edge 58. The curved anterior edge 57 begins to curveaway from the left edge 58 at the lower tier 52 and gradually rampsupward along with the sloped portion 54 as it moves toward the rightedge 56. A posterior edge 59 is substantially perpendicular to the rightedge 56 and left edge 58. The posterior edge 59 extends between theright edge 56 and left edge 58 along curved corner portions 60 and 61 ofthe lower tier 52. In one alternative embodiment, the posterior edgecomprises a sloped portion, similar to the anterior sloped portion 54.

Sidewalls 62, 64, 66 and 68 define the sides of the superior plate 22and extend between the upper surface 40 and lower surface 50. The rightsidewall 62 is best seen in FIGS. 1-3, the left sidewall 66 andposterior sidewall 68 are best seen in FIG. 6, and the anterior sidewall64 is best seen in FIG. 7. Along with the upper surface 40 and lowersurface 50, the sidewalls 62, 64, 66, and 68 define the shape and volumeof the superior endplate 22.

As noted by the dotted lines in FIG. 1 associated with the superiorendplate 22, a cylindrical cavity 70 is formed in the superior endplate22. The cavity 70 provides a generally cylindrical hole in the superiorendplate 22. An opening to the cavity 70 is formed in the lower surface50 of the superior endplate 22. The elastic member 24 is designed to fitwithin the cavity 70 as shown in FIGS. 3 and 4. The elastic member 24may be secured in the cavity 70 in numerous ways such as mechanicallyfastened in the cavity, friction fit in the cavity, or bonded within thecavity. For example, an adhesive material may be used to secure theelastic member 24 within the cavity 70. Alternatively, a chemical bondmay be formed between the superior plate 22 and the elastic member 24.

The elastic member 24 is designed to provide a resilient cushion betweenthe superior endplate 22 and the inferior endplate 28 while alsoallowing some limited movements between the endplates 22 and 28. Theelastic member 24 disclosed herein is generally cylindrical in shape.The elastic member is formed from an elastomeric material havingresilient qualities. For example, in one embodiment, the elastic member24 may be comprised of polyurethane. As another example, the elasticmember 24 may be comprised of silicone.

The elastic member 24 includes a lower disc portion 74 and an upper ribportion. The upper rib portion includes six equally spaced ribs 72 thatextend from a central axis of the elastic member 24. Void spaces areprovided between the ribs 72. Each rib 72 may be deformed when exposedto a threshold force. For example, when a force is applied to a rib 72it may bend into the void space provided next to the rib. When the forceis removed, the ribs resiliently move back to their original shape.

As shown by the dotted line on the elastic member 24 of FIG. 1, a domedcavity 76 is formed in the elastic member 24. The opening to the domedcavity 76 is provided on the bottom portion of the elastic member 24.The domed cavity 76 is designed to receive the dome-shaped bearingmember 26. The dome-shaped bearing member 26 is secured within thecavity 76 of the elastic member 24 using any of numerous means known inthe art. For example, a mechanical fastener may be used to secure thebearing member in the cavity 76 or an adhesive may be used to bond thebearing member 26 in the cavity 76. Alternatively, a chemical bond maybe formed between the elastic member 24 and the dome-shaped bearingmember 26.

The dome-shaped bearing member 26 is comprised of a wear-resistantmaterial such as a metal, ceramic or a hardened polymer. In oneembodiment, the bearing 26 is comprised of cobalt chromium. Thedome-shaped bearing 26 is generally spherical in shape and includes aconvex upper surface 27 and a concave lower surface 25 (see FIG. 4). Theconvex upper surface 27 is designed to fit within the cavity 76 of theelastic member 24. A fastener or bond may be used to secure the bearing26 to the elastic member 24. The concave lower surface 25 of the bearing26 is configured to be congruent with and rotatably engage a ball-shapedbearing 31 provided on the inferior endplate 28. Because the dome-shapedbearing 26 is secured to the elastic member 24 which, in turn, issecured to the superior plate 22, the dome-shaped bearing 26 provides abearing surface for rotation of the superior plate 22 relative to theinferior plate 28.

With continued reference to FIG. 1, the inferior plate 28 is comprisedof a rigid material such as a rigid metal, ceramic or a rigid polymer.The inferior plate may comprise a single molded piece or a multi-piececomponent. In one embodiment, the inferior plate comprises asubstantially flat upper surface 32 with a ball-shaped bearing 31 formedon the upper surface 32. The ball-shaped bearing 31 is centrallypositioned upon the upper surface 32 slightly posterior of a lateralmidline. The ball-shaped bearing 31 provides a substantially sphericalbearing surface 30 designed and dimensioned to rotatably engage theconcave lower surface 25 of the dome-shaped bearing 26. The concavelower surface 25 of the dome-shaped bearing 26 essentially provides asocket for the spherical bearing surface 30, resulting in a ball andsocket arrangement between the superior endplate 22 and the inferiorendplate 28.

The center of the bearing 31 defines a center of rotation for the firstprosthetic device 20. In particular, with the superior endplate 22positioned on the inferior endplate 28 and the spherical bearing surface30 engaging the convex lower surface of the dome-shaped bearing 26, thesuperior plate 22 is adapted to rotate about the center of the bearing31. Thus, the spherical bearing surface 30 and the dome-shaped bearing26 form an articulating joint, and particularly a ball and socket joint,between the superior plate 22 and the inferior plate 28.

The inferior plate 28 further comprises a lower surface 34 designed toface the inferior vertebra of the intervertebral space. A lower keel 36is positioned on the lower surface 34 and extends below the lowersurface 34. The keel 36 is configured to fit in a channel formed in aninferior vertebra and secure the inferior endplate 28 to the inferiorvertebra. An alignment hole 38 is provided in the keel 36. As will beexplained in further detail below, the alignment hole 38 is designed toassist the surgeon in aligning the first prosthetic device 20 with asecond prosthetic device. In an alternative embodiment, teeth areprovided on the endplate surfaces in place of the upper and lower keel.Such teeth may be used for fixing the endplates on the vertebral bodiesand may also be used to assist in proper alignment of the endplates.

Arrangement of First and Second Prosthetic Devices

As stated above, the first prosthetic device 20 is configured for usewith a complimentary second prosthetic device 120. The second prostheticdevice is a mirror image of the first prosthetic device shown in FIGS.1-4. The first and second prosthetic devices 20 and 120 are designed forside-by-side arrangement within an intervertebral space. Such aside-by-side arrangement is now described with reference to FIGS. 5-7.

FIG. 5 shows the footprint of the inferior endplate 28 of the firstprosthetic device 20 positioned on the left side of an inferiorvertebral body 200. The inferior endplate 128 of the secondcomplimentary prosthetic device 120 is positioned on the right side ofthe vertebral body 200. The vertebral body 200 includes a disc region201, a spinal foramen 202, and a plurality of facet joints. A lateralmidline of the vertebral body 200 is shown by line 210. The lateralmidline 210 is line that extends from left to right across the vertebralbody and is perpendicular to a longitudinal center line 211 extendingbetween an extreme anterior position and an extreme posterior positionon the vertebral body. While the lateral midline 210 is intended toextend through a center of the disc region 201, it should be understoodthat numerous lines extending from left to right through a central pointof the disc region 210 may be considered lateral midlines. For example,in one embodiment, the lateral midline is a line slightly to theposterior of the actual center of the disc region 201.

As suggested by the footprints of the inferior endplates 28 and 128 uponthe vertebral body 200 in FIG. 5, the first prosthetic device 20 andsecond prosthetic device 120 are symmetrically aligned upon thevertebral body 200 about the longitudinal center line 211. Furthermore,the center of rotation 33 of the first prosthetic device 20 is alignedwith the center of rotation 133 of the complimentary second prostheticdevice 120 along the lateral midline 210 or slightly posterior to thelateral midline 210 of the vertebral bodies. The centers of rotation 33and 133 are shown in FIG. 5 projected on to the vertebral body. Becausethe intervertebral disc prosthesis is a two-piece component with a leftprosthesis device 20 and a right prosthesis device 120, the prosthesiscan be easily implanted in a patient from a posterior approach. Inparticular, the left prosthesis device 20 may be inserted from the leftside of the spinal cord 202 and the right prosthesis device 120 may beinserted from the right side of the spinal cord 202. During theimplantation procedure, it may be necessary to remove facet joints tofacilitate the posterior approach. While facet joints limit the degreeof torsional movement, the intervertebral disc prosthesis describedherein also limits torsional movement, as will be discussed in furtherdetail below. Therefore, complications resulting from removed facetjoints are compensated by the intervertebral disc prosthesis describedherein. However, in some circumstances it may be desirable to usestabilizer devices on the facet joints to further limit the amount oftorsional movement.

FIG. 6 provides a rear perspective view of the first prosthetic device20 aligned next to the complimentary second prosthetic device 120. Asshown in FIG. 6, the alignment hole 44 in the upper keel of the firstprosthetic device 20 is aligned with the alignment hole 144 of thesecond prosthetic device 120. Similarly, the alignment hole 38 of thelower keel of the first prosthetic device 20 is aligned with thealignment hole 138 of the lower keel of the second prosthetic device.During implantation of the first and second prosthetic devices, thesurgeon may advantageously use the alignment holes to confirm precisealignment of the first prosthetic device 20 relative to the secondprosthetic device. In particular, a lateral x-ray image of the first andsecond prosthetic devices during the implantation procedure shouldreveal holes 38 and 138 in alignment along a lateral midline and asimilar alignment between holes 44 and 144.

FIG. 7 provides a front view of the first prosthetic device 20positioned side-by-side with the second prosthetic device 120. As shownin FIG. 7 (and considering FIG. 4), the anterior sidewall 64 of thefirst superior endplate 22 curves around and upward from the leftsidewall 66 and tapers into the junction of the right sidewall 62 andthe upper surface 40 of the endplate 22 at an extreme forward positionof the endplate. Similarly, the anterior sidewall 164 of the secondsuperior endplate 122 curves around and upward from the left sidewall162 and tapers into the junction of the right sidewall 166 and the uppersurface 140 of the endplate 122 at an extreme forward position of theendplate. In each case the anterior sloped portion 54 and 154 of thelower surface of the endplate can be seen from the front view.

With reference to FIG. 8, in one embodiment, the first prosthetic device20 and second prosthetic device 120 include a connecting member spanningbetween the first prosthetic device and the second prosthetic device.For example, as shown in FIG. 8, a connection rod 90 may extend betweenthe first superior plate 22 and the second superior plate 122. Thisconnection rod 90 joins the first superior plate 22 to the secondsuperior plate 122, insuring that the first superior plate 22 remainsaligned with the second superior plate 122 and move in unison as asingle rigid component. In particular, the connection rod 90 prohibitsdrifting of one endplate relative to the other following implantation ofthe endplates in the patient.

Prosthesis Movement

When the first prosthetic device 20 and second prosthetic device 120 areimplanted in an intervertebral space, the intervertebral disc prosthesisgenerally allows a significant degree of flexion/extension movement buta lesser degree of torsional and lateral bending movement. Withreference to FIGS. 5-7, flexion/extension movement is represented byarrows 220 about lateral midline 210. Because superior endplates 22 and122 are both secured to the same superior vertebral body, they will movein unison. Likewise, inferior endplates 28 and 128 are secured to thesame inferior vertebral body and will move in unison. The center ofrotation for each prosthetic device is provided along line 210, which isthe line about which flexion and extension occur. Accordingly,significant degrees of flexion and extension are provided as indicatedby arrow 220 about line 210 as the center of rotation of both the firstprosthetic device 20 and the second prosthetic device 120 are positionedalong line 210. Extension is limited to a degree at which the superiorplates 22 and 122 rotate posteriorly into contact with the inferiorplates 28 and 128. Likewise, flexion is limited to a degree at which thesuperior plates 22 and 122 rotate anteriorly into contact with theinferior plates 28 and 128. It should be noted that substantially moreflexion is allowed than extension because of the anterior slopedportions 54 and 154 on the superior plates 22 and 122 which provideadditional clearance between the superior plates 22 and 122 and theinferior plates 28 and 128 during anterior rotation of the plates. Inone embodiment, allowed degrees of extension may be as much as 5° or 10°while allowed degrees of flexion may be as much as 10° or 20°.

The intervertebral disc prosthesis also allows some degree of lateralbending as noted by arrow 221 in FIG. 5 indicating rotation aboutlongitudinal center line 211. The degree of lateral bending issubstantially dependent upon the deformation properties of the elasticmembers 24 and 124 of the respective prosthetic devices 20 and 120. Inparticular, during lateral bending the superior vertebra and inferiorvertebra attempt to rotate about longitudinal center line 211 of FIG. 5.However, because centers of rotation 33 and 133 are removed from thelongitudinal center line 211, these points act as supports thatgenerally resist lateral bending. Nevertheless, some degree of lateralbending is possible due to the deformation properties of the elasticmembers 24 and 124. For example, as shown in FIG. 7 dotted line 230indicates a degree θ of leftward lateral bending from the equilibriumposition indicated by line 220. During this degree of leftward lateralbending, the elastic member 24 in the left prosthesis device 20 iscompressed as the left superior endplate 22 rotates relative to the leftinferior endplate 28. In addition to compression of the elastic member,a small amount of pivoting may occur between the left bearing surfaces25 and 30. If the leftward lateral bending force is large enough, thebearing surface retained in the right superior endplate 122 may actuallydisengage and pull away from the ball bearing of the right inferiorendplate 128. Of course, the bearing surfaces return to full engagementfollowing removal of such force. Accordingly, the intervertebral discprosthesis described herein provides for relatively limited degrees oflateral bending movement. In one embodiment, lateral bending asrepresented by angle θ in FIG. 7 is generally limited to a range from 2°to 6°.

In addition to the movements described above, the intervertebral discprosthesis also allows a limited amount of axial rotation (torsional)movement, as indicated by arrow 222 in FIG. 5. Arrow 222 suggests somemovement about an axis 212 that is perpendicular to axes 210 and 211.However, rotation in this torsional direction 222 is limited as thefirst prosthetic device 20 and the second prosthetic device 120 providetwo fixed points on axis 210, as noted by centers of rotation 33 and 133in FIG. 5. Therefore, the only torsional movement of the superiorendplates 22 and 122 relative to the inferior endplates 28 and 128results from compression of the elastic members 24 and 124 between theplates. For example, during torsional movement to the left, the anteriorportion of left elastic member 24 is compressed, while the posteriorportion of right elastic member 124 is compressed. The degree of axialrotation allowed is dictated by the deformation properties of theelastic members 24 and 124 and the force imposed upon the elasticmembers. In one embodiment, the degree of axial rotation is limited to arange from 1° to 3°.

Alternative embodiment

With reference now to FIGS. 9-13, an alternative embodiment of aprosthetic device 300 is shown. Similar to the prosthetic devicesdescribed above, the prosthetic device 300 is designed for use as oneside of a two-component intervertebral disc prosthesis.

With general reference to FIGS. 9-12, an exploded side view of a rightside first prosthetic device 320 for an intervertebral disc prosthesis300 is shown. The first prosthetic device 320 comprises a superiorendplate 322, an elastic member 324, a dome-shaped bearing member 326,and an inferior endplate 328. The superior endplate 322 is configured toretain the elastic member 324, and the elastic member 324 is configuredto retain the dome-shaped bearing member 326. The dome-shaped bearingmember 326 is configured to engage a spherical bearing surface 330 ofthe inferior endplate 328.

As shown in FIGS. 9-11, the superior endplate 322 includes an uppersurface 340 designed to face a superior vertebra in an intervertebralspace. While not shown in the figures, fixation devices such as teeth ora keel may be used to secure the superior endplate 322 to the superiorvertebra. The footprint of the superior endplate 322 is generallykidney-shaped with a generally convex curvature on the right perimeter341 and a generally concave curvature along the left perimeter 343. Atab 345 extends downward from the left perimeter 343.

The superior endplate 322 also includes a lower surface 350 opposite theupper surface 340. The lower surface 350 includes a circular mouth 352that defines a cavity 370 designed to receive the elastic member 324.The mouth 352 comprises a frusto-conical sidewall that extends downwardto an opening to the cavity 370. The elastic member 324 is designed tofit within the cavity 370. The elastic member 324 may be secured in thecavity in numerous ways such as mechanical fasteners, snap-fit, orbonded within the cavity.

The elastic member 324 is designed to provide a resilient cushionbetween the superior endplate 322 and the inferior endplate 328 whileallowing some limited movements between the endplates 322 and 328. Theelastic member 324 is generally cylindrical in shape and is formed froman elastomeric material having resilient qualities. The elastic member324 includes a lower disc portion 374 and an upper rib portion 375. Theupper rib portion 375 includes six equally spaced ribs 372 that extendfrom a central axis of the elastic member 324. Each rib 372 may bedeformed when exposed to a threshold force. When the force is removed,the ribs 372 resiliently move back to their original shape.

As shown by the dotted line on the elastic member 324 of FIG. 9, a domedcavity 376 is formed in the elastic member 324. The opening to the domedcavity 376 is provided on the bottom portion 374 of the elastic member324. The domed cavity 376 is designed to receive the dome-shaped bearingmember 326. The dome-shaped bearing member 326 is secured within thecavity 376 of the elastic member 324 using a fastener or other meanssuch as a bond.

The dome-shaped bearing member 326 is comprised of a wear-resistantmaterial such as a metal, ceramic or a hardened polymer. The dome-shapedbearing 326 is generally spherical in shape and includes a convex uppersurface 327 and a concave lower surface (not shown). The convex uppersurface 327 is designed to fit within the cavity 376 of the elasticmember 324. A fastener, bond or other means may be used to secure thebearing 326 to the elastic member 324. The concave lower surface of thebearing 326 is configured to be congruent with and rotatably engage aspherical bearing surface 330 provided on the inferior endplate 328.Because the dome-shaped bearing 326 is secured to the elastic member 324which, in turn, is secured to the superior plate 322, the dome-shapedbearing 326 provides a bearing surface for rotation of the superiorplate 322 relative to the inferior plate 328.

With continued reference to FIGS. 9-12, the inferior plate 328 is formedof a rigid material such as a rigid metal, ceramic or a rigid polymer.The inferior plate 328 comprises a substantially flat upper surface 332with a ball-shaped bearing 331 formed on the upper surface 332. Theball-shaped bearing 331 is centrally positioned upon the upper surface332 slightly posterior of a lateral midline. The ball-shaped bearing 331provides a substantially spherical bearing surface 330 designed anddimensioned to rotatably engage the concave lower surface of thedome-shaped bearing 326. The concave lower surface of the dome-shapedbearing 326 essentially provides a socket for the spherical bearingsurface 330, resulting in a ball and socket arrangement between thesuperior endplate 322 and the inferior endplate 328.

The center point of the bearing 331 defines a center of rotation for thefirst prosthetic device 320. In particular, with the superior endplate322 positioned on the inferior endplate 328 and the spherical bearingsurface 330 engaging the convex lower surface of the dome-shaped bearing326, the superior plate 322 is adapted to rotate relative to theinferior plate 328 about the center of rotation.

Also included on the upper surface 332 of the inferior endplate 328 is aposterior upper tier 336. The posterior upper tier 336 provides asurface that restricts the degree of extension available to the superiorplate 322 relative to the inferior plate 328.

The inferior plate 328 further comprises a lower surface 334 designed toface the inferior vertebra of the intervertebral space. While not shownin the figures, fixation devices such as teeth or a keel may be used tosecure the inferior endplate 328 to the inferior vertebra. The footprintof the inferior endplate 328 is generally kidney-shaped with a generallyconvex curvature on the right perimeter 351 and a generally concavecurvature along the left perimeter 353.

As stated previously, the first prosthetic device 320 is configured foruse with a complimentary second prosthetic device 420. The secondprosthetic device 420 is a mirror image of the first prosthetic deviceshown in FIGS. 9-12. The first and second prosthetic devices 320 and 420are designed for side-by-side arrangement within an intervertebralspace. FIG. 13 shows the footprint of the inferior endplate 328 of thefirst prosthetic device 320 positioned on the right side of an inferiorvertebral body 200. The inferior endplate 428 of the secondcomplimentary prosthetic device 420 is positioned on the left side ofthe vertebral body 200. The vertebral body 200 includes a disc region201, a spinal foramen 202, and a plurality of facets. A lateral midlineof the vertebral body 200 is shown by line 210.

As suggested by the footprints of the endplates 328 and 428 upon thevertebral body 200 in FIG. 13, the first prosthetic device 320 andsecond prosthetic device 420 are symmetrically aligned upon thevertebral body 200 about the longitudinal center line 211. Furthermore,the center of rotation 333 of the first prosthesis 320 is aligned withthe center of rotation 433 of the complimentary second prosthesis 420along the lateral midline 210. Because the intervertebral discprosthesis is a two-piece component with a right prosthesis device 320and a left prosthesis device 420, the prosthesis can be easily implantedin a patient from a posterior approach. Furthermore, as explainedpreviously, the intervertebral disc prosthesis allows significantdegrees of flexion/extension movement, but substantially limited degreesof lateral bending and torsional movement.

In an alternative configuration not shown in the figures, the firstprosthetic device 320 and the second prosthetic device 420 include aconnection member, such as additional bars or other links joining thefirst prosthetic device and the second prosthetic device, similar to thearrangement shown in FIG. 8. For example, two superior endplates of thefirst prosthetic device 320 and the second prosthetic device 420 may berigidly connected and fixed to one another such that movement of oneendplate causes movement of the other endplate. Therefore, the superiorendplates may form a single rigid body and the inferior endplates mayform a single rigid body.

Another alternative configuration shown in FIG. 14. In this embodiment,the intervertebral disc prosthesis 500 comprises a single superior plate510 and a single inferior plate (not shown) with multiple articulatingjoints positioned between the superior plate 510 and the inferior plate.The articulating joints are particularly a first ball and socket jointand a second ball and socket joint. The first ball and socket jointdefines a first center of rotation 520 and the second ball and socketjoint defines a second center of rotation 530. The first center ofrotation 520 and the second center of rotation 530 are shown projectedon the superior plate 510 in FIG. 14 and are provided along a linesubstantially parallel to the lateral midline 210. To facilitate thefirst ball and socket joint and the second ball and socket joint, afirst socket and a second socket are both provided in the superior plate510, as suggested by circular dotted lines 540 and 550 in FIG. 14.Similarly, the inferior endplate includes a first ball bearing and asecond ball bearing positioned which define the first center of rotationand the second center of rotation for the prosthesis 500. An elastomer,such as elastic member 24 described above is provided between thesuperior endplate and the inferior endplate in a similar fashion to thatdescribed above. The prosthesis 500 is designed for insertion in thepatient using either a lateral approach or an anterior approach.

Although the present invention has been described with respect tocertain preferred embodiments, it will be appreciated by those of skillin the art that other implementations and adaptations are possible. Forexample, the elastic member could be positioned between the inferiorendplate and the spherical bearing surface rather than between thesuperior endplate and the dome-shaped bearing surface. Also, the elasticmember could be positioned between both the superior endplate and thedome-shaped bearing surface and the inferior endplate and the sphericalbearing surface. In such embodiments, the inferior endplate may includea cavity to receive the elastic member configured for connection to thespherical bearing surface. As another example of an alternativeembodiment, the spherical bearing surface may be positioned on thesuperior endplate rather than the inferior endplate. In such embodiment,the inferior endplate may define a concave bearing surface that receivesthe spherical bearing surface attached to the superior endplate. In yetanother example of an alternative embodiment, the superior endplate, theelastic member and the dome-shaped bearing surface may be provided as amulti-durometer component formed by various molding processes, such asmulti-shot injection molding, where the endplate, elastic member andbearing surface are provided as single integral part. Of course numerousother alternative embodiments are possible. Moreover, there areadvantages to individual advancements described herein that may beobtained without incorporating other aspects described above. Therefore,the spirit and scope of the appended claims should not be limited to thedescription of the preferred embodiments contained herein.

1. An intervertebral disc prosthesis designed for implantation betweenan upper vertebral body and a lower vertebral body, the intervertebraldisc prosthesis comprising: a) a first prosthesis component including afirst superior vertebra facing surface, a first inferior vertebra facingsurface, and a substantially spherical first bearing surface positionedbetween the first superior vertebra facing surface and the firstinferior vertebra facing surface, wherein the first superior vertebrafacing surface is operable to rotate relative to the first inferiorvertebra facing surface upon the first bearing surface; and b) a secondprosthesis component including a second superior vertebra facingsurface, a second inferior vertebra facing surface and a substantiallyspherical second bearing surface positioned between the second superiorvertebra facing surface and the second inferior vertebra facing surface,wherein the second superior vertebra facing surface is operable torotate relative to the second inferior vertebra facing surface upon thesecond bearing surface.
 2. The intervertebral disc prosthesis of claim 1wherein the first bearing surface defines a first center of rotation andthe second bearing surface defines a second center of rotation, andwherein the first center of rotation and second center of rotation arepositioned along a line substantially parallel to a lateral midline ofthe upper vertebral body or the lower vertebral body.
 3. Theintervertebral disc prosthesis of claim 1 wherein the first prosthesiscomponent is connected to the second prosthesis component.
 4. Theintervertebral disc prosthesis of claim 3 wherein the first superiorvertebra facing surface is connected to the second superior vertebrafacing surface.
 5. The intervertebral disc prosthesis of claim 3 whereinthe first inferior vertebra facing surface is connected to the secondinferior vertebra facing surface.
 6. The intervertebral disc prosthesisof claim 1 further comprising a first elastic member positioned betweenthe first superior vertebra facing surface and the first inferiorvertebra facing surface and a second elastic member positioned betweenthe second superior vertebra facing surface and the second inferiorvertebra facing surface.
 7. The intervertebral disc prosthesis of claim6 wherein the first elastic member is positioned between the firstsuperior vertebra facing surface and the first bearing surface andwherein the second elastic member is positioned between the secondsuperior vertebra facing surface and the second bearing surface.
 8. Theintervertebral disc prosthesis of claim 6 wherein the first elasticmember is positioned between the first inferior vertebra facing surfaceand the first bearing surface and wherein the second elastic member ispositioned between the second inferior vertebra facing surface and thesecond bearing surface.
 9. An intervertebral disc prosthesis designedfor implantation between an upper vertebral body and a lower vertebralbody, the intervertebral disc prosthesis comprising: a) a leftprosthesis component including a first left vertebra facing surface, asecond left vertebra facing surface, and a left bearing surfacepositioned between the first left vertebra facing surface and the secondleft vertebra facing surface, wherein the first left vertebra facingsurface is operable to rotate relative to the second left vertebrafacing surface upon the left bearing surface, the left prosthesiscomponent further comprising a left elastic member positioned betweenthe first left vertebra facing surface and the left bearing surface; andb) a right prosthesis component including a first right vertebra facingsurface, a second right vertebra facing surface, and a right bearingsurface positioned between the first right vertebra facing surface andthe second right vertebra facing surface, wherein the first rightvertebra facing surface is operable to rotate relative to the secondright vertebra facing surface upon the right bearing surface, the rightprosthesis component further comprising a right elastic memberpositioned between the first right vertebra facing surface and the rightbearing surface.
 10. The intervertebral disc prosthesis of claim 9wherein the left bearing surface defines a left center of rotation forthe left prosthesis component, wherein the right bearing surface definesa right center of rotation for the right prosthesis component, andwherein the left center of rotation and the right center of rotation arearranged on a line substantially parallel to the lateral midline of theupper vertebral body or the lower vertebral body.
 11. The intervertebraldisc prosthesis of claim 9 wherein the first left vertebra facingsurface is a left superior vertebra facing surface, the second leftvertebra facing surface is a left inferior vertebra facing surface, thefirst right vertebra facing surface is a right superior vertebra facingsurface, and the second right vertebra facing surface is a rightinferior vertebra facing surface.
 12. The intervertebral disc prosthesisof claim 9 wherein the left bearing surface and right bearing surfaceare substantially spherical.
 13. The intervertebral disc prosthesis ofclaim 9 wherein the left elastic member and right elastic member arecomprised of an elastomeric material.
 14. The intervertebral discprosthesis of claim 9 wherein the first left vertebra facing surface isprovided on a first left plate, the second left vertebra facing surfaceis provided on a second left plate, the first right vertebra facingsurface is provided on a first right plate, and the second rightvertebra facing surface is provided on a second right plate.
 15. Theintervertebral disc prosthesis of claim 14 wherein a left cavity isformed in the first left plate and a right cavity is formed in the firstright plate, wherein the left elastic member and left bearing surfaceare positioned in the left cavity and the right elastic member and rightbearing surface are positioned in the right cavity.
 16. Theintervertebral disc prosthesis of claim 15 further comprising a leftdome member having a concave surface and a right dome member having aconcave surface, wherein the concave surface of the left dome member isprovided in the left cavity and engages the left bearing surface, andwherein the concave surface of the right dome member is provided in theright cavity and engages the right bearing surface.
 17. Theintervertebral disc prosthesis of claim 9 wherein the left elasticmember is substantially cylindrical in shape and the right elasticmember is substantially cylindrical in shape.
 18. The intervertebraldisc prosthesis of claim 9 wherein the left elastic member comprises aplurality of voids and the right elastic member comprises a plurality ofvoids.
 19. The intervertebral disc prosthesis of claim 18 wherein theleft elastic member further comprises a plurality of ribs formed betweenthe plurality of voids in the left elastic member, and the right elasticmember further comprises a plurality of ribs formed between theplurality of voids in the right elastic member.
 20. An intervertebraldisc prosthesis comprising: a) a first plate; b) a second plate; c) abearing surface positioned between the first plate and the second plate,wherein the first plate is configured to rotate relative to the secondplate upon the bearing surface; and d) an elastic member positionedbetween the first plate and the bearing surface, wherein the elasticmember is configured to retain the bearing surface and the first plateis configured to retain the elastic member.
 21. The intervertebral discprosthesis of claim 20 wherein the first plate is a superior plate andthe second plate is an inferior plate.
 22. The intervertebral discprosthesis of claim 20 wherein the bearing surface is a dome-shapedmember having a concave inner surface configured to engage a sphericalsurface on the second plate.
 23. The intervertebral disc prosthesis ofclaim 20 wherein the elastic member comprises a plurality of voids. 24.The intervertebral disc prosthesis of claim 20 wherein the elasticmember comprises a plurality of deformable ribs.
 25. The intervertebraldisc prosthesis of claim 20 wherein the elastic member is comprised ofan elastomer.
 26. The intervertebral disc prosthesis of claim 20 whereinthe first plate is a first left plate, the second plate is a second leftplate, the bearing surface is a left bearing surface, and the elasticmember is a left elastic member.
 27. The intervertebral disc prosthesisof claim 26 further comprising a first right plate, a second rightplate, a right bearing surface positioned between the first right plateand the second right plate, wherein the first right plate is configuredto rotate relative to the second right plate upon the right bearingsurface, and a right elastic member positioned between the first rightplate and the right bearing surface, wherein the right elastic member isconfigured to retain the right bearing surface and the first right plateis configured to retain the right elastic member.
 28. The intervertebraldisc prosthesis of claim 27 wherein the first right plate is connectedto the first left plate.
 29. The intervertebral disc prosthesis of claim27 wherein the second right plate is connected to the second left plate.30. An intervertebral disc prosthesis configured for placement between asuperior vertebra and an inferior vertebra, the intervertebral discprosthesis comprising: a) a superior vertebra facing surface; b) aninferior vertebra facing surface; c) a plurality of ball and socketjoints positioned between the superior vertebra facing surface and theinferior vertebra facing surface, the plurality of ball and socketjoints configured to facilitate rotation of the superior vertebra facingsurface relative to the inferior vertebra facing surface.
 31. Theintervertebral disc prosthesis of claim 30 wherein the superior vertebrafacing surface comprises a first portion provided by a first superiorendplate and a second portion provided by a second superior endplate,wherein the first endplate is fixed to the second endplate.
 32. Theintervertebral disc of claim 31 further comprising a first elasticmember positioned between the first superior endplate and a first of theplurality of ball and socket joints and a second elastic member providedbetween the second superior endplate and a second of the plurality ofball and socket joints.
 33. The intervertebral disc prosthesis of claim30 wherein the inferior vertebra facing surface comprises a firstportion provided by a first inferior endplate and a second portionprovided by a second inferior endplate, wherein the first endplate isfixed to the second endplate.
 34. The intervertebral disc of claim 33further comprising a first elastic member positioned between the firstinferior endplate and a first of the plurality of ball and socket jointsand a second elastic member provided between the second inferiorendplate and a second of the plurality of ball and socket joints.